AO shadows ind Cycles
The Ambient Occlusion shader can look unrealistic because unlike most other cycles shaders, it is not based on how light interacts with materials in the real world.

Instead, the lightness of shadows depends on how far other objects are. If it "sees" an object close by, it assumes that part of the mesh should bee in complete darkness. Otherwise, it produces a lighter color. If the ray tracer reaches no objects at all and reaches the sky, it produces very bright colors.

While unrealistic, AO has it's uses. It will usually give you less noise than the diffuse shader, whose behavior it very loosely imitates. This means that you can sometimes get away with fewer samples (and hence a smaller render time) by using AO.

However, this will not give you a realistic rendering. If you are willing to use more samples and want a photorealistic picture, use the Diffuse shader instead.

You can compensate between realism and quick rendering by mixing AO and Diffuse using the Mix node:



AO only



50% AO, 50% Diffuse



Diffuse only



Notice that mixing the AO with diffuse, you can reduce some of the unnatural brightness resulting from AO.